A communication scheme for continuously maintaining communication connection without imposing any unwanted load on a network has been proposed conventionally. In the conventional technique, disconnection can be prevented by periodically transmitting packets (Keep-Alive). As, for example, a client autonomously decides an appropriate Keep-Alive interval, connection is continuously maintained without imposing any unwanted load on a network (see, for example, Japanese Patent Laid-Open No. 2003-18235).
On the other hand, a push communication scheme of performing communication past a proxy server by continuously maintaining connection from a server on the Internet to a client on an intranet located inside a firewall has been proposed (see, for example, “The WebSocket protocol: draft-ietf-hybi-thewebsocketprotocol-17,” [online], Sep. 30, 2011, IETF, [Searched on Dec. 27, 2011], Internet <URL:http://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-17>).
In Japanese Patent Laid-Open No. 2003-18235, appropriate Keep-Alive transmission is impossible when communication is performed past a firewall by push communication as shown in “The WebSocket protocol: draft-ietf-hybi-thewebsocketprotocol-17,” [online], Sep. 30, 2011, IETF, [Searched on Dec. 27, 2011], Internet <URL:http://tools.ietf.org/html/draft-ietf-hybi-thewebsocketprotocol-17>. This indicates that in push communication, a communication device such as a firewall is present in a communication path, so a server cannot perform Keep-Alive transmission to a client apparatus on an intranet at an appropriate interval. As a result, a communication apparatus in a communication path may disconnect connection, so communication from the server on the Internet to the client apparatus becomes impossible.
Also, if the Keep-Alive interval is short, the amount of excess communication increases, so the amount of traffic increases, and the communication efficiency lowers. However, if the Keep-Alive interval is long, communication interruption occurs due to the presence of a firewall, and connection cannot be maintained, so instant data transfer becomes impossible.